252 Minnesota Algae 



and Flahault. Revis. des Nostoc. Ann. Sci. Nat. Bot. VII. 5: 80. 

 1887. be Toni. Syll. Algar. 5: 592. 1907. 

 Farlow. Notes on Fresh-Water Algae. Bot. Gaz. 8: 225. 1883. Col- 

 lins, Holden and Setchell. Phyc. Bor.-Am. Fasc. 3. no. no. 1895. Setchell. 

 Notes on some Cyanophyceae of New England. Bull. Torr. Bot. Club. 22: 

 427. 1895. Collins. Phycological Notes of the late Isaac Holden. — II. 

 Rhodora. 7: 237. 1905. 



Plate XVI. fig. 2. 



Colony vesicular, lobed, up to 2 cm. in diameter, hollow, blue-green 

 or yellowish green; trichomes 4-9 mic. in diameter, I mm. in length, branch- 

 ed from the base, loose, elongate, fiexuous, often constricted at joints; 

 branches unilateral, fastigiate, cylindrical below, torulose in upper portions, 

 somewhat club-shaped; cells up to twice as long as wide; heterocysts 

 lateral, exserted, or intercalary. 



Vermont. Forming expansions of several inches in water courses. Fer- 

 risburg. (Faxon and Hosford). Connecticut. Forming irregular, firmly 

 gelatinous balls growing upon stones in more or less rapid water in a 

 brook just west of the "head" of the mountain. Mt. Carmel, about seven 

 miles north of New Haven. September 1893 and 1895. (Setchell). In brook. 

 Mt. Carmel. September. (Holden). Pennsylvania. Floating. Schuylkill 



River, just above Manayunk. (Wood). 



Family V. RIVULARIACEAE 



Filaments tapering from base to apex, ending in a colorless hair, simple 

 or branched; false branches due to development of new trichome from a 

 cell of the main trichome, usually occurring immediately under an inter- 

 calary heterocyst — rarely by the perforation of the sheath between two 

 heterocysts by the trichome, as in Scytonema — either separating immediate- 

 ly and forming a new sheath, or remaining for some time within the origi- 

 nal sheath; heterocysts usually present, usually basal, occasionally inter- 

 calary; reproduction by means of vegetative division, hormogones and 

 gonidia. 



I Heterocysts not present Amphithrix 



II Heterocysts present 



I Filaments free, simple or coalesced into a branched plant mass 

 (i) Sheaths cylindrical 

 A Filaments simple or branched; false branches distinct, free 



Calothrix 

 B Filaments branched; false branches several (two to six) remain- 

 mg within the original sheath or common tegument 



Dichothrix 

 C Filaments branched; .false branches many (up to a hundred) re- 

 maining within the original sheath or common tegument 



Polythrix 

 (2) Sheaths thick, saccate Sacconema 



